Want the default border to be none!

I can't seem to figure this out and have run out of time to search the various forums etc. My apologies if it is covered somewhere!

When I work in Numbers I like to keep borders off until I am done with the table I am creating. When I drag a cell however, it leaves behind a grey set of border lines which seem to be the default border style. I can't find a place to change the default style and it is VERY irritating to have borders cropping up because I simply drag the contents of a cell or a range of cells to another area of the table.

Does anyone know how to avoid this??